If you genuinely believe Trump to have fomented insurrection and to be unfit to serve, then you vote to impeach him and deal with tomorrow, tomorrow.
What he believes is that his legislative agenda is his work, and that he's the best person to advance the agenda and stymie the opposition.
In his mind, Trump is a sideshow to that. Trump's unpopularity is likely to be enduring, and if Congress can convict him subsequent to a term of office then they can wait to move on Trump until he runs for office and only if the election is competitive.
The case of William Blount also is relevant. Congress had him dead to rights on a treason charge
in his own handwriting and was unable to act beyond expelling Blount from the Senate. All politics is tribal.
So I don't think it's as clear cut as you make it. If enough of the Republican conference thought the way you do, it's an easy call to do as you suggest and we're not having this conversation. However, the fact that the Republicans tried to use Trump rather than remove him prior to this, which is 100% on McConnell, has left McConnell in his present predicament.